Image formation by the electrophotographic method is generally performed through a process including: forming an electrostatic image on a photoconductor (latent electrostatic image bearing member); developing the electrostatic image with a developer to form a visible image (toner image); transferring the visible image onto a recording medium such as paper; and fixing the transferred image on the recording medium to form a fixed image (see, for example, PTL 1).
In recent years, from the viewpoint of saving energy, development has been made on technology capable of forming a toner having lower fixing temperature. For example, there have been proposed toners containing a low-softening-point resin, wax, etc. excellent in low-temperature fixability.
Also, there have been proposed capsule toners composed of a hard shell and a core softened at low temperatures. These capsule toners are excellent in low-temperature fixability but are poor in durability, and thus have not used practically. In view of this, there have recently been proposed toners containing a crystalline resin (e.g., a crystalline polyester) having a sharp response to heat, instead of the capsule toners composed of a hard shell and a core softened at low temperatures (see, for example, PTLs 2 and 3).
By improving toners in low-temperature fixability, there can surely be produced toners that respond to fixing at low temperatures. However, the toners excellent in low-temperature fixability tend to involve blocking phenomenon in which the toners are hardened due to, for example, heat generated from the apparatus or during storage, resulting in that they are problematically poor in heat resistant storage stability.
In addition, there are concerns that the toners are pulverized by stress such as stirring in the developing device to cause toner spent and/or filming on the developing member, carrier, etc. In order to overcome such failures and incorporate a crystalline polyester into the toner in a certain amount or more, the toners have to be encapsulated. However, the core containing a crystalline polyester in a certain amount or more is soft. Thus, the capsule toner obtained by encapsulating such a soft core with a shell has a problem in that it is poor in durability similar to the aforementioned capsule toners composed of a hard shell and a soft core.
In order for the toner to be excellent in all of low-temperature fixability, heat resistant storage stability and developing stability, there has been a toner in which the amount of deformation at pressing with 1 mN is 1.0 μm to 3.0 μm and the amount of deformation at pressing with 5 mN is 3.0 μm to 5.0 μm, which are measured by a deformation evaluation method, and the surface roughness Ra is 0.02 μm to 0.40 μm as measured by a Ra evaluation method (see PTL 4).
However, although this proposed toner is excellent in low-temperature fixability and heat resistant storage stability, it is not satisfactory in durability to stress in the developing device such as stirring, which is problematic.
In view of the above, demand has arisen for a toner excellent in low-temperature fixability and heat resistant storage stability as well as having sufficient durability to stress in the developing device such as stirring; and a developer, an image forming apparatus and an image forming method each using the toner.